System and method to determine thermophysical properties of a multi-component gas at arbitrary temperature and pressure

ABSTRACT

A method to determine thermodynamic properties of a natural gas hydrocarbon, when the speed of sound in the gas is known at an arbitrary temperature and pressure. Thus, the known parameters are the sound speed, temperature, pressure, and concentrations of any dilute components of the gas. The method uses a set of reference gases and their calculated density and speed of sound values to estimate the density of the subject gas. Additional calculations can be made to estimate the molecular weight of the subject gas, which can then be used as the basis for mass flow calculations, to determine the speed of sound at standard pressure and temperature, and to determine various thermophysical characteristics of the gas.

RELATED APPLICATION

This application is a continuation-in-part application of U.S. patentapplication Ser. No. 09/550,431 filed Apr. 17, 2000, entitled, “Systemand Method to Determine Thermophysical Properties of a Multi-ComponentGas”, now U.S. Pat. No. 6,604,051.

GOVERNMENT LICENSE RIGHTS

The U.S. Government has a paid-up license in this invention and theright in certain circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of Contract No.DE-FC21-96MC33033 for the U.S. Department of Energy.

TECHNICAL FIELD OF THE INVENTION

This invention relates to the field of measuring thermophysicalproperties of gases, and more particularly, to the determination ofthermophysical gas properties using gross inferential properties andempirical correlations, at arbitrary temperature and pressure.

BACKGROUND OF THE INVENTION

Virtually all natural gas produced in the U.S. is used as fuel. As such,its intrinsic value lies in its heating value. However, because of thelack of low-cost, reliable instrumentation for measuring energy contentor energy flow rate, natural gas traditionally has been bought, sold,and transported on the basis of volume. Gas composition and energycontent are currently determined by periodically analyzing sampled gasfrom the pipeline.

With the advent of deregulation and open access in the gas pipelineindustry, large-volume, long-term commodity gas contracts gave way tomore small-volume, short-term transportation contracts, taking gas“packages” from many supply and storage fields with widely varying gasqualities. These gas packages lose their identity when mixed in thepipeline, and the purchaser receives whatever is in the pipeline at thetime of need.

Without economical means for continuously (real-time or near real-time)measuring the quality of gas entering and exiting the pipeline, neitherthe supplier nor the end-user can assure quality of the commodityexchange. Also, gas supplies cannot be blended to assure conformity to aquality standard. End-users withdraw gas on the basis of energy needs.If the energy content is low, end-users simply withdraw (and pay for)more gas than anticipated.

Current technology provides two approaches to energy flow ratemeasurement for natural gas. The first requires a composition assay anda flow rate measurement. The composition assay allows calculating theheating value of the gas, and is also required to calculate selected gasproperties (e.g., gas density) needed to determine energy flow rates.The second approach measures gas density and heating value directly,using special instrumentation, and requires no composition assay.

SUMMARY OF THE INVENTION

The system and method of the present invention provide for determinationof thermophysical properties of a multi-component hydrocarbon gas.Exploiting the interdependence of properties the components of such agas allows characterizing its hydrocarbon energy without a detailedcomposition assay.

The method described herein involves determining the diluentconcentrations (e.g., predominantly nitrogen and carbon dioxide) ofthese gas. These concentrations may be measured or inferred from othermeasurements with approximate accuracy, as they account for only a smallfraction of the whole natural gas mixture. The remaining hydrocarbon gascomponents (i.e., the majority of the gas mixture) can be characterizedby inferential properties, without differentiation of species. For thepurpose of the present invention, three inferential properties wereselected in an exemplary fashion. These are the speed of sound, carbondioxide concentration, and nitrogen concentration. The speed of soundmay be measured at any arbitrary temperature and pressure.

As a specific example, the concentration of carbon dioxide and nitrogenare determined, along with the speed of sound in the gas, to determine athermophysical property (e.g. the Mixture Molar Ideal Gross HeatingValue). An empirical correlation exists between the thermophysicalproperty, the speed of sound, the concentration of carbon dioxide, andthe concentration of nitrogen in the gas.

Depending on the gas components for which the concentration isdetermined, various thermophysical properties can be determined more orless accurately. For example, the Mixture Molar Ideal Gross HeatingValue, the Mixture Molecular Weight, the Mass-Based Heating Value, andthe Density of the gas can all be determined within about ±0.02% ofselected model values by implementing the system and method of thepresent invention.

The concentration of the diluent gas components may be determineddirectly (e.g., via measurement), or indirectly. For example, theconcentration of a particular gas component may be determined bycorrelating a thermodynamic property for the selected component with oneor more directly measurable inferential properties of the component.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the apparatus of the present invention.

FIG. 2 is a flow diagram illustrating the method of determiningthermophysical properties of a gas according to the present invention.

FIG. 3 illustrates an alternative embodiment of the method of thepresent invention, which is based on properties of the gas at anyarbitrary temperature and pressure.

FIG. 4 further illustrates the method of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

System Overview

FIG. 1 illustrates the apparatus of the present invention. The system 10for determining thermophysical properties of a gas is typicallyinstalled on a gas pipeline. The pipe 20 is shown in a cut-away viewcarrying a multi-component gas 30 (or, using alternative terminology, agas having a plurality of components 30). Typically, such componentsinclude nitrogen 31, carbon dioxide 32, methane 33, ethane 34, propane35, and heavier hydrocarbons 36, each component existing within the gas30 in varying proportions.

System 10 includes a sound velocity measurement device 40 adapted tomeasure the speed of sound in the gas 30. This device is preferably aninexpensive ultrasonic flow meter, such as those produced by Siemens forresidential applications in the European gas market, e.g. the Siemens E6domestic gas meter. In addition, any of the following meters aresuitable for use as a sound velocity measurement device 40 in the gas30: Daniel Senior Sonic, Junior Sonic, and UltraTap ultrasonic flowmeters; the Smith Meter Kongsberg MPU1200 ultrasonic flow meter; thePanametrics Ultrasonic Transit-Time flow meter; and the Instromet QSonic-2S, -3S, and -5S ultrasonic flow meters. The sound velocitymeasurement device 40 may also comprise an array of ultrasonic sensorsand associated electronics configured specifically to measure the speedof sound, as is well known in the art.

Also comprising an element of system 10 is a concentration measurementdevice 50 adapted to determine the concentration of a plurality ofcomponents comprising the gas 30. The concentration measurement device50 can include a commercial, non-dispersive infrared sensor, which maybe similar to or identical to the Vaisala model GMM11, which measuresthe concentration of carbon dioxide using infrared energy absorption.Other suitable instruments which may comprise the device 50 and whichcan be used to determine component concentrations include sensors ormeters to directly measure the concentration of nitrogen, carbondioxide, and/or other components.

As an alternative approach, the concentration measurement device 50 maybe used to determine a gas concentration inferentially, in combinationwith measurements of sound speed and another gas componentconcentration. Examples include the inferential determination of N₂ byany of the following methods: measuring the sound speed at two differentpressures and temperatures, measuring heat dissipation and temperaturerise in a container having a fixed volume, measuring heat dissipationand temperature rise in a gas stream flowing at a constant rate, orindependently measuring the gas density.

The sound velocity measurement device 40 and the concentrationmeasurement device 50 are typically connected so as to be in electricalcommunication with the processor 80 using cables 60. A correlationdevice 55 may also comprise a part of the concentration measurementdevice, or reside as an independent correlation device 55 outside themeasurement device 50, such as in the processor 80. The correlationdevice 55 may be incorporated as a software module within the processor80, or as a separate hardware module, such as a Digital Signal Processor(DSP) or some other computational device dedicated to effecting thecorrelation calculations described hereinafter. That is, thecomputational load on the processor 80 may be divided between theprocessor 80 and the correlation device 55. However, in certainapplications, there is no need for this type of division, and theprocessor 80 may be suitable for gathering data from the various sensorsand making the correlation computations necessary to carry out themethod of the present invention. Typically, the processor 80 will have amemory 90 for storing the results of sensor measurements, programminginformation, and calculation results for chemical properties of the gas30.

As can be seen in FIG. 1, the measurements of concentration and speed ofsound can be made within the pipe 20 at flowing temperature andpressure. Alternatively, the measurements can be made within a gassample, outside of the pipe, held in a sample chamber 65, for example.In this case, the gas may be sampled from within the pipe 20 by using asample tube 62 which directs the gas sample into a sample chamber 65.Thus, any one or more of the measurements noted above can be made withinthe sample chamber 65 using the concentration measurement device 50, thesound velocity measurement device 40, and/or the natural gas flow meter70. Measurements of any of these properties may be made within the pipe20, within the sample chamber 65, or from a combination of the twolocations. A preferred embodiment makes use of an existing ultrasonicflow meter 40 to measure the sound speed in the pipeline 20 at theflowing gas temperature and pressure, and a gas sample stream drawn offof the pipeline 20 through a sample tube 62 to measure the nitrogen andcarbon dioxide concentrations.

Natural gas flow meters 70 may be generally classified as volume-based(ultrasonic, turbine, rotary, diaphragm, etc.), differential-based(orifice, annular, V-cone, etc.), and mass-based (Coriolis,thermal-mass, etc.). Any of these meters 70 may be used for energymeasurement, but each meter classification requires the determination ofdifferent properties to complete the energy flow rate calculation. Forexample, volume-based meters require: $\begin{matrix}{Q_{{energy}{{volume}\text{-}{based}}} = {{Q_{v,{std}}H_{v,{std}}} = {{( \frac{\rho \quad Q_{v}}{\rho_{std}} )( {\rho_{std}H_{m}} )} = {{Q_{v}( {\rho \quad H_{m}} )} = {Q_{v}H_{v}}}}}} & \text{[1-1]}\end{matrix}$

Flow Rates

To complete the energy flow rate calculation, volume-based metersrequire only the volume-based heating value, H_(v) [Btu/acf], which isthe product of flowing density, ρ[lbm/ft³], and mass-based heatingvalue, H_(m) [Btu/lbm]. If traditional formulations of standardvolumetric flow rate, Q_(v,std)[scf/h], and heating value,H_(v,std)[Btu/scf], are desired, then standard density, ρ_(std)[typically measured in lbm/ft³ at 60° F., 14.73 psia, at the flowingcomposition], is also needed.

Energy flow rate through a differential (pressure-based) meter requires:$\begin{matrix}{Q_{{energy},{{differential}\text{-}{based}}} = {{Q_{v,{std}}H_{v,{std}}} = {{( \frac{C\sqrt{\rho}}{\rho_{std}} )( {\rho_{std}H_{m}} )} = {C( {\sqrt{\rho}\quad H_{m}} )}}}} & \text{[1-2]}\end{matrix}$

Note that differential meters require determination of the flowing gasdensity, ρ[lbm/ft³], and the mass-based heating value, H_(m)[Btu/lbm].If traditional formulations of standard volumetric flow rate,Q_(v,std)[scf/h], and heating value, H_(v,std)[Btu/scf], are desired,then standard density, ρ_(std) [typically lbm/ft³ at 60° F., 14.73 psia,at the flowing composition], is also needed. Note that the catch-allfactor C depends on the viscosity, μ (through the dischargecoefficient), and on the isentropic exponent, κ (through the expansionfactor), but these are of lesser importance.

Energy flow rate calculation using a mass-based meter requires:$\begin{matrix}{Q_{{energy},{{mass}\text{-}{based}}} = {{Q_{v,{std}}H_{v,{std}}} = {{( \frac{Q_{m}}{\rho_{std}} )( {\rho_{std}H_{m}} )} = {Q_{m}\quad H_{m}}}}} & \text{[1-3]}\end{matrix}$

Only the addition of mass-based heating value, H_(m) [Btu/lbm], isrequired to determine energy flow rate through a mass-based meter. Iftraditional formulations of standard volumetric flow rate, Q_(v,std)[scf/h], and heating value, H_(v,std) [Btu/scf], are desired, then thestandard density, ρ_(std), will also be needed to convert from amass-basis to a standard-volume basis.

Equations [1-1] to [1-3] describe the property dependence of differentmeter classes with respect to energy measurement, but all properties arenot equally difficult to determine. Chemical properties depend only ongas composition (chemical characterization). Thermodynamic propertiesdepend on gas composition, temperature, and pressure. Furthermore,thermodynamic properties can become chemical properties (dependent onlyon gas composition) at a fixed temperature and pressure, although thechemical dependence will vary at different temperature and pressureconditions.

Thus, three natural gas properties are critical to energy measurement.Two chemical properties, H_(m) and ρ_(std), are required by all threemeter classes, and an additional thermodynamic property (ρ) is requiredby volume-based and differential-based meters. Mass-based meters areuniquely independent properties, an important advantage that makesflowing gas temperature and pressure irrelevant for this class ofmeters.

Other chemical properties, in addition to H_(m) and ρ_(std), are alsouseful. Mixture Molecular Weight, M [lbm/lb-mol], is useful as a datacorrelation intermediary to characterize gas composition, and may alsobe used to calculate Ideal Specific Gravity. Mixture Molar Ideal GrossHeating Value, H_(n,ref) [kJ/mol, with chemical-enthalpies referenced to25° C., 0.101325 MPa], is a useful intermediary to characterizing gascomposition, especially gas density, ρ, from an existing equation ofstate.

The basis of inferential property determination is chemicalcharacterization. Chemical properties are constant across gas-phasetemperature and pressure conditions. In contrast, thermodynamicproperties must be chemically characterized for selected temperature andpressure conditions.

Reference Gas Mixtures

For the purpose of defining various chemical properties and derivingdata correlations, a database of 102 different natural gas compositionswas developed. Of the first 51 compositions, 43 are unique. Theremaining 51 compositions can be obtained by reversing the carbondioxide and nitrogen concentrations of the first 51 compositions. Thecomplete gas composition database is presented in Table 1.

TABLE I nitrogen carbon methane [mole dioxide [mole ethane propanel-butane n-butane l-pentane n-pentane n-hexane n-heptane n-octane %][mole %] %] [mole %] [mole %] [mole %] [mole %] [mole %] [mole %] [mole%] [mole %] [mole %] 1 0.4422 1.9285 84.3361 8.8946 3.1919 0.590640.39376 0.10950 0.07300 0.0325 0.0061 0.0012 2 1.6004 0.2331 95.53401.8790 0.4926 0.09066 0.06044 0.03522 0.02348 0.0309 0.0168 0.0034 35.6769 1.4546 85.1473 5.4174 1.5968 0.30438 0.20292 0.08694 0.057960.0391 0.0138 0.0019 4 0.6224 1.8643 85.4814 8.0607 2.8624 0.521700.34780 0.11346 0.07564 0.0398 0.0096 0.0008 5 5.4939 1.8292 84.39315.8857 1.6910 0.32742 0.21828 0.06942 0.04628 0.0296 0.0128 0.0033 65.3551 1.7802 84.4786 5.8782 1.7778 0.34002 0.22668 0.07050 0.047000.0309 0.0116 0.0034 7 1.6052 0.2339 95.5192 1.8835 0.4933 0.091080.06072 0.03546 0.02364 0.0326 0.0176 0.0038 8 0.4278 1.8497 84.46788.8604 3.1831 0.58008 0.38672 0.11994 0.07996 0.0369 0.0068 0.0008 95.6760 1.4579 85.1666 5.4022 1.5922 0.30366 0.20244 0.08706 0.058040.0385 0.0134 0.0020 10 0.6122 1.8630 85.4915 8.0626 2.8576 0.522540.34836 0.11412 0.07608 0.0404 0.0100 0.0016 11 1.6052 0.2339 95.51921.8835 0.4933 0.09108 0.06072 0.03546 0.02364 0.0326 0.0176 0.0038 120.4278 1.8497 84.4678 8.8604 3.1831 0.58008 0.38672 0.11994 0.079960.0369 0.0068 0.0008 13 5.6760 1.4579 85.1666 5.4022 1.5922 0.303660.20244 0.08706 0.05804 0.0385 0.0134 0.0020 14 0.6122 1.8630 85.49158.0626 2.8676 0.52254 0.34836 0.11412 0.07608 0.0404 0.0100 0.0016 151.6032 0.2299 95.5480 1.8724 0.4883 0.08982 0.05988 0.03462 0.023080.0312 0.0164 0.0032 16 0.4293 1.8647 84.4333 8.8669 3.1897 0.581820.38788 0.12066 0.08044 0.0377 0.0072 0.0004 17 5.6680 1.4349 85.17845.4163 1.5962 0.30426 0.20284 0.08706 0.05804 0.0387 0.0131 0.0022 180.6137 1.8710 85.4620 8.0768 2.8634 0.52272 0.34848 0.11412 0.076080.0405 0.0101 0.0011 19 2.3535 0.0401 92.2794 3.7252 0.9170 0.261660.17444 0.08898 0.05932 0.0654 0.0115 0.0235 20 2.6733 0.0402 93.03573.1217 0.6420 0.16896 0.11264 0.06930 0.04620 0.0436 0.0229 0.0235 212.4630 1.5280 90.8251 4.4050 0.6420 0.06774 0.04516 0.01386 0.009240.0003 0.0006 0.0000 22 0.4040 1.9870 83.9520 9.1380 3.2590 0.593400.39560 0.12540 0.08360 0.0473 0.0139 0.0008 23 0.4050 2.0270 83.86819.1800 3.2790 0.58560 0.39040 0.12294 0.08196 0.0459 0.0136 0.0005 240.3940 1.9730 83.7500 93.490 3.3080 0.58080 0.38720 0.11976 0.079840.0447 0.0133 0.0004 25 1.2630 1.9820 88.9650 5.4550 1.6160 0.307800.20520 0.09780 0.06520 0.0310 0.0120 0.0000 26 4.1950 1.5730 87.98104.8020 0.9080 0.18840 0.12560 0.09300 0.06200 0.0450 0.0260 0.0010 271.9080 1.9860 92.7220 2.7990 0.3430 0.06180 0.04120 0.06420 0.042800.0150 0.0170 0.0000 28 5.1240 0.5810 88.8020 4.1500 0.8580 0.179400.11960 0.07320 0.04880 0.0400 0.0220 0.0020 29 4.9480 1.6030 86.64604.9600 1.2440 0.24180 0.16120 0.08640 0.05760 0.0340 0.0170 0.0010 300.4230 2.1250 84.0050 8.7790 3.2380 0.64740 0.43160 0.16740 0.111600.0590 0.0130 0.0000 31 2.4750 1.7790 87.9700 5.5520 1.5120 0.295200.19680 0.09840 0.06560 0.0360 0.0190 0.0010 32 5.5400 1.7960 86.44504.7560 0.9140 0.19860 0.13240 0.08580 0.05720 0.0460 0.0260 0.0030 332.5050 0.9750 92.3210 3.2850 0.5690 0.11100 0.07400 0.06000 0.040000.0350 0.0230 0.0020 34 4.1230 0.7040 90.4400 3.5110 0.7500 0.170400.11360 0.07140 0.04760 0.0410 0.0250 0.0030 35 1.0370 2.0360 88.04806.2390 1.8390 0.36780 0.24520 0.09120 0.06080 0.0260 0.0080 0.0020 360.6122 1.8630 85.4915 8.0626 2.8576 0.52254 0.34836 0.11412 0.076080.0404 0.0100 0.0016 37 0.6137 1.8710 85.4620 8.0768 2.8634 0.522720.34848 0.11412 0.07608 0.0405 0.0101 0.0011 38 0.6178 1.9051 85.34538.1433 2.8692 0.53850 0.35900 0.10470 0.06980 0.0345 0.0117 0.0011 393.7924 0.2609 94.6077 1.0118 0.2128 0.04572 0.03048 0.01464 0.009760.0086 0.0044 0.0008 40 0.9015 0.0668 98.2722 0.5159 0.1607 0.035520.02368 0.00942 0.00628 0.0055 0.0016 0.0009 41 0.4313 1.7708 85.45608.4983 2.7421 0.53706 0.35804 0.10038 0.06692 0.0315 0.0068 0.0008 425.3551 1.7802 84.4784 5.8782 1.7780 0.34002 0.22668 0.07050 0.047000.0309 0.0116 0.0034 43 5.4939 1.8292 84.3931 5.8857 1.6910 0.327420.21828 0.06942 0.04628 0.0296 0.0128 0.0033 44 5.3452 1.7745 84.51435.8831 1.7596 0.33582 0.22388 0.07044 0.04696 0.0309 0.0119 0.0034 455.4952 1.8318 84.3746 5.8795 1.7111 0.32880 0.21920 0.06906 0.046040.0297 0.0117 0.0033 46 0.9617 1.5021 85.9284 8.4563 2.3022 0.419100.27940 0.07308 0.04872 0.0228 0.0057 0.0005 47 0.4264 1.9201 84.37898.8749 3.1776 0.60132 0.40088 0.10872 0.07248 0.0310 0.0065 0.0012 481.2010 1.8560 88.2210 6.1190 1.8840 0.35340 0.23560 0.05580 0.037200.0230 0.0130 0.0010 49 0.3407 1.8816 83.4187 9.5284 3.5694 0.621900.41460 0.10968 0.07312 0.0327 0.0081 0.0011 50 5.9990 1.3984 84.48725.9271 1.5364 0.30534 0.20356 0.06342 0.04228 0.0251 0.0101 0.0021 511.4200 0.0330 93.3240 1.7800 3.2000 0.08700 0.05800 0.02520 0.016800.0560 0.0000 0.0000 52 1.9285 0.4422 84.3361 8.8946 3.1919 0.590640.39376 0.10950 0.07300 0.0325 0.0061 0.0012 53 0.2331 1.6004 95.53401.8790 0.4926 0.09066 0.06044 0.03522 0.02348 0.0309 0.0168 0.0034 541.4546 5.6769 85.1473 5.4174 1.5968 0.30438 0.20292 0.08694 0.057960.0391 0.0138 0.0019 55 1.8643 0.6224 85.4814 8.0607 2.8624 0.521700.34780 0.11346 0.07564 0.0398 0.0096 0.0008 56 1.8292 5.4939 84.39315.8857 1.6910 0.32742 0.21828 0.06942 0.04628 0.0296 0.0128 0.0033 571.7802 5.3551 84.4786 5.8782 1.7778 0.34002 0.22668 0.07050 0.047000.0309 0.0116 0.0034 58 0.2339 1.6052 95.5192 1.8835 0.4933 0.091080.06072 0.03546 0.02364 0.0326 0.0176 0.0038 59 1.8497 0.4278 84.46788.8604 3.1831 0.58008 0.38672 0.11994 0.07996 0.0369 0.0068 0.0008 601.4579 5.6760 85.1666 5.4022 1.5922 0.30366 0.20244 0.08706 0.058040.0385 0.0134 0.0020 61 1.8630 0.6122 85.4915 8.0626 2.8576 0.522540.34836 0.11412 0.07608 0.0404 0.0100 0.0016 62 0.2339 1.6052 95.51921.8835 0.4933 0.09108 0.06072 0.03546 0.02364 0.0326 0.0176 0.0038 631.8497 0.4278 84.4678 8.8604 3.1831 0.58008 0.38672 0.11994 0.079960.0369 0.0068 0.0008 64 1.4579 5.6760 85.1666 5.4022 1.5922 0.303660.20244 0.08706 0.05804 0.0385 0.0134 0.0020 65 1.8630 0.6122 85.49158.0626 2.8576 0.52254 0.34836 0.11412 0.07608 0.0404 0.0100 0.0016 660.2299 1.6032 95.5480 1.8724 0.4883 0.08982 0.05988 0.03462 0.023080.0312 0.0164 0.0032 67 1.8647 0.4293 84.4333 8.8669 3.1897 0.581820.38788 0.12066 0.08044 0.0377 0.0072 0.0004 68 1.4349 5.6680 85.17845.4163 1.5962 0.30426 0.20284 0.08706 0.05804 0.0387 0.0131 0.0022 691.8710 0.6137 85.4620 8.0768 2.8634 0.52272 0.34848 0.11412 0.076080.0405 0.0101 0.0011 70 0.0401 2.3535 92.2794 3.7252 0.9170 0.261660.17444 0.08898 0.05932 0.0654 0.0115 0.0235 71 0.0402 2.6733 93.03573.1217 0.6420 0.16896 0.11264 0.06930 0.04620 0.0436 0.0229 0.0235 721.5280 2.4630 90.8251 4.4050 0.6420 0.06774 0.04516 0.01386 0.009240.0003 0.0006 0.0000 73 1.9870 0.4040 83.9520 9.1380 3.2590 0.593400.39560 0.12540 0.08360 0.0473 0.0139 0.0008 74 2.0270 0.4050 83.86819.1800 3.2790 0.58560 0.39040 0.12294 0.08196 0.0459 0.0136 0.0005 751.9730 0.3940 83.7500 9.3490 3.3080 0.58080 0.38720 0.11976 0.079840.0447 0.0133 0.0004 76 1.9820 1.2630 88.9650 5.4550 1.6160 0.307800.20520 0.09780 0.06520 0.0310 0.0120 0.0000 77 1.5730 4.1950 87.98104.8020 0.9080 0.18840 0.12560 0.09300 0.06200 0.0450 0.0260 0.0010 781.9860 1.9080 92.7220 2.7990 0.3430 0.06180 0.04120 0.06420 0.042800.0150 0.0170 0.0000 79 0.5810 5.1240 88.8020 4.1500 0.8580 0.179400.11960 0.07320 0.04880 0.0400 0.0220 0.0020 80 1.6030 4.9480 86.64604.9600 1.2440 0.24180 0.16120 0.08640 0.05760 0.0340 0.0170 0.0010 812.1250 0.4230 84.0050 8.7790 3.2380 0.64740 0.43160 0.16740 0.111600.0590 0.0130 0.0000 82 1.7790 2.4750 87.9700 5.5520 1.5120 0.295200.19680 0.09840 0.06560 0.0360 0.0190 0.0010 83 1.7960 5.5400 86.44504.7560 0.9140 0.19860 0.13240 0.08580 0.05720 0.0460 0.0260 0.0030 840.9750 2.5050 92.3210 3.2850 0.5690 0.11100 0.07400 0.06000 0.040000.0350 0.0230 0.0020 85 0.7040 4.1230 90.4400 3.5110 0.7500 0.170400.11360 0.07140 0.04760 0.0410 0.0250 0.0030 86 2.0360 1.0370 88.04806.2390 1.8390 0.36780 0.24520 0.09120 0.06080 0.0260 0.0080 0.0020 871.8630 0.6122 85.4915 8.0626 2.8576 0.52254 0.34836 0.11412 0.076080.0404 0.0100 0.0016 88 1.8710 0.6137 85.4620 8.0768 2.8634 0.522720.34848 0.11412 0.07608 0.0405 0.0101 0.0011 89 1.9051 0.6178 85.34538.1433 2.8692 0.53850 0.35900 0.10470 0.06980 0.0345 0.0117 0.0011 900.2609 3.7924 94.6077 1.0118 0.2128 0.04572 0.03048 0.01464 0.009760.0086 0.0044 0.0008 91 0.0668 0.9015 98.2722 0.5159 0.1607 0.035520.02368 0.00942 0.00628 0.0055 0.0016 0.0009 92 1.7708 .04313 85.45608.4983 2.7421 0.53706 0.35804 0.10038 0.06692 0.0315 0.0068 0.0008 931.7802 5.3551 84.4784 5.8782 1.7780 0.34002 0.22668 0.07050 0.047000.0309 0.0116 0.0034 94 1.8292 5.4939 84.3931 5.8857 1.6910 0.327420.21828 0.06942 0.04628 0.0296 0.0128 0.0033 95 1.7745 5.3452 84.51435.8831 1.7596 0.33582 0.22388 0.07044 0.04696 0.0309 0.0119 0.0034 961.8318 5.4952 84.3746 5.8795 1.7111 0.32880 0.21920 0.06906 0.046040.0297 0.0117 0.0033 97 1.5021 0.9617 85.9284 8.4563 2.3022 0.419100.27940 0.07308 0.04872 0.0228 0.0057 0.0005 98 1.9201 0.4264 84.37898.8749 3.1776 0.60132 0.40088 0.10872 0.07248 0.0310 0.0065 0.0012 991.8560 1.2010 88.2210 6.1190 1.8840 0.35340 0.23560 0.05580 0.037200.0230 0.0130 0.0010 100 1.8816 0.3407 83.4187 9.5284 3.5694 0.621900.41460 0.10968 0.07312 0.0327 0.0081 0.0011 101 1.3984 5.9990 84.48725.9271 1.5364 0.30534 0.20356 0.06342 0.04228 0.0251 0.0101 0.0021 1020.0330 1.4200 93.3240 1.7800 3.2000 0.08700 0.05800 0.02520 0.016800.0560 0.0000 0.0000

Model Equations for Thermodynamic Properties

The values for M, H_(m), ρ_(std), and H_(n,ref.) can be calculated usingthe model equations shown below, and the composition values from Table Ifor each natural gas composition. $\begin{matrix}{M = {\sum\limits_{i = 1}^{N}( {\frac{X_{i}}{100}M_{i}} )}} & \text{[1-4]}\end{matrix}$

where N is the number of pure gas components in the mixture, X_(i) isthe mole percentage of each component, and M_(i) is the molecular weightof each component (from the Gas Processor's Association (GPA) Standard2145 (1994)). $\begin{matrix}{H_{m} = \frac{\sum\limits_{i = 1}^{N}( {\frac{X_{i}}{100}M_{i}H_{m,i}} )}{M}} & \text{[1-5]}\end{matrix}$

where N is the number of pure gas components in the mixture, X_(i) isthe mole percentage of each component, M_(i) is the molecular weight ofeach component, and H_(m,i) is the mass-based heating value of eachcomponent (from the GPA) Standard 2145 (1994). In the natural gasindustry, it is standard practice to use chemical enthalpies (heatingvalues) determined at standard conditions of 60° F. and 14.696 psia,even though the volumetric basis is often converted to a pressure of14.73 psia. The GPA Standard 2145 (1994) provides the H_(m,i) data atthe accepted chemical enthalpy conditions of 60° F. and 14.696 psia.$\begin{matrix}{\rho_{std} = \frac{P_{std}M}{{ZRT}_{std}}} & \text{[1-6]}\end{matrix}$

In this case, the real gas state equation is used, where P_(std) andT_(std) are standard pressure and temperature (absolute units), M is themixture molecular weight, Z is the compressibility factor, and R is theuniversal gas constant. Since P_(std), T_(std), and R are all constants,ρ_(std) is a function of only the variable ratio M/Z. $\begin{matrix}{H_{n,{ref}} = {\sum\limits_{i = 1}^{N}( {\frac{X_{i}}{100}H_{n,{ref},i}} )}} & \text{[1-7]}\end{matrix}$

where X_(i) is the mole percentage of each pure gas component, andH_(n,ref,i) is the molar ideal gross heating value (at 25° C. and0.101325 MPa) of each pure gas component (from American Gas Association(A.G.A.) Transmission Committee Report 8 (1994)).

Assuming three inferential gas characterization properties are known,namely, the speed of sound at 60° F. and 14.73 psia, S_(std)[ft/s], thecarbon dioxide concentration, X_(CO2) [mol %], and the nitrogenconcentration, X_(N2)[mol %], and that the Mixture Molecular Weight, M,has a quadratic relationship with respect to the standard sound speed,S_(std), such that the S_(std) can be calculated using the commerciallyavailable SonicWare™ software or the GRI Extended ThermodynamicProperties Computer Programs (Gas Research Institute 1989, unpublished)at 60° F. and 14.73 psia, the form of the inferential correlation may bechosen as:

M=A+BS _(std) +CS _(std) ²  [1-8 ]

where: A=A₀+A₁X_(N2)+A₂X_(CO2), B=B₀+B₁X_(N2)+B₂X_(CO2), andC=C₀+C₁X_(N2)+C₂X_(CO2). A least squares curve fit of the data in Table1 produces the following values for the unknown constants: A₀=89.59987,A₁=0.2595616, A₂=0.8420112, B₀=−0.08303539, B₁=−3.57614×10⁻⁴,B₂=−1.20199×10⁻³, C₀=2.22787×10⁻⁵, C₁=1.37342×10⁻⁷, and C₂=4.51462×10⁻⁷.

Assuming knowledge of the same inferential gas characterizationproperties, that H_(m,CO2)=H_(m,N2)=0 (i.e., that diluent gases have noheating value), and that H_(m) is a weak function of hydrocarboncomposition, the Hydrocarbon Molecular Weight, M_(HC), may be definedas: $\begin{matrix}{M_{HC} = {M - ( {\frac{X_{CO2}}{100}M_{CO2}} ) - ( {\frac{X_{N2}}{100}M_{N2}} )}} & \text{[1-9]}\end{matrix}$

And the form of the inferential correlation may be chosen as:$\begin{matrix}{H_{m} = ( \frac{A + {BM}_{HC}}{M} )} & \text{[1-10]}\end{matrix}$

where: B=B₀+B₁S_(std) and B₁=B₂+B₃X_(N2)+B₄X_(CO2). A least squarescurve fit of the data in Table 1 produces values for the unknownconstants: A=54,343.048, B₀=20,442.406, B₂=0.04552871, B₃=−0.02523803,and B₄=−0.02568212.

Even though gas density is, in general, a thermodynamic property, thestandard density is a chemical property because it is evaluated atspecific conditions of 60° F., 14.73 psia, and at the flowing gascomposition.

Because M has already been correlated to inferential properties (Seeequation 1-8]), and Z varies only about 0.1% across the Table 1 databasecompositions due to the low-pressure standard condition, the form of thestandard density correlation is straightforward. Therefore, the samecorrelation form as equation [1-8] may be used to determine uniquecorrelation constants for standard density:

ρ_(std) =A+BS _(std) +CS _(std) ²  [1-11]

where: A=A₀+A₁X_(N2)+A₂X_(CO2), B=B₀+B₁X_(N2)+B₂X_(CO2), andC=C₀+C₁X_(N2)+C₂X_(CO2). A least squares fit to the database of Table 1produced values for the unknown constants: A₀=0.2395147,A₁=7.067074×10⁻⁴, A₂=2.334917×10⁻³, B₀=−2.228333×10⁻⁴, B₁=−9.87042×10⁻⁷,B₂=−3.35135×10⁻⁶, C₀=5.99480×10⁻⁸, C₁=3.81330×10⁻¹⁰, andC₂=1.26106×10⁻⁹.

The Molar Ideal Gross Heating Value, at 25° C. and 0.101325 MPareference conditions, is important as an intermediate variable that maybe used to characterize natural gas composition for densitycalculations.

The form of the data correlation to inferential variables is chosen tobe the same as that for the mass-based heating value, H_(m), which isalso a molar ideal gross heating value at a different reference state,using different units:

H _(n,ref) =A+BM _(HC)  [1-12]

where: B=B₀+B₁S_(std), and B₁=B₂+B₃X_(N2)+B₄X_(CO2). A least squarescurve fit of the data in the Table 1 database produces values for theunknown constants: A=123.81271, B₀=47.41274, B₂=2.73661×10⁻⁴,B₃=−5.71187×10⁻⁵, B₄=−5.73574×10⁻⁵.

Thermodynamic properties are more difficult to characterize thanchemical properties because chemical property dependence must be modeledas a function of temperature and pressure. As discussed with respect tovarious flow meters, gas density, ρ, is the most important thermodynamicproperty related to natural gas energy flow measurement. The isentropicexponent, κ, and viscosity, μ, are required for differential-basedmeters, but are less important because they form a part of lesssensitive terms (the isentropic exponent is needed for the expansionfactor, and viscosity is needed for the Reynolds number). Thus, it isimportant to be able to correlate gas density, ρ, to the pre-selectedinferential properties of standard sound speed, S_(std), carbon dioxideconcentration, X_(CO2), nitrogen concentration, X_(N2), and, as notedabove, the temperature, T, and pressure, P.

The U.S. natural gas industry currently has two well-accepted equationsof state for computing natural gas density (i.e., the compressibilityfactor). These are contained in the A.G.A. Transmission MeasurementCommittee Report 8 (1994), referenced and included as if set forthentirely herein, and denominated as: the Detailed CharacterizationMethod, which requires a detailed gas composition assay to characterizethe gas, and the Gross Characterization Method, which requiresinferential properties to characterize the gas. Both methods specify anuncertainty level of 0.1% over 32-130° F., up to 1200 psia, and gascompositions in the “normal range” detailed in Table 2.

TABLE 2 Quantity Normal Range Relative Density* 0.554-0.87  GrossHeating Value** [Btu/scf]   447-1,150 Gross Heating Value***[MJ/m³]18.7-45.1 Methane [mol %]  45.0-100.0 Nitrogen [mol %]   0-50.0 CarbonDioxide [mol %]   0-30.0 Ethane [mol %]   0-10.0 Propane [[mol %]  0-4.0 Total Butanes [mol %]   0-1.0 Total Pentanes [mol %]   0-0.3Hexanes Plus [mol %]   0-0.2 Helium [mol %]   0-0.2 Hydrogen [mol %]  0-10.0 Carbon Monoxide [mol %]   0-3.0 Water [mol %]   0-0.05 HydrogenSulfide [mol %]   0-0.02 *Reference Condition: Relative density at 60°F., 14.73 psia **Reference Conditions: Combustion at 60° F., 14.73 psia;density at 60° F., 14.73 psia. ***Reference Conditions: Combustion at25° C., 0.101325 MPa; density at 0° C., 0.101325 MPa.

The current Gross Characterization Method allows the user tocharacterize a gas using one of two inferential variable sets:volumetric gross heating value, relative density, and carbon dioxideconcentration (Method 1); or relative density, carbon dioxideconcentration, and nitrogen concentration (Method 2). This inventionintroduces a new variable set: standard sound speed, S_(std), carbondioxide concentration, X_(CO2), and nitrogen concentration, X_(N2)(designated hereinafter as “Method 3”).

The Gross Characterization Method is a virial equation of state, and iscast in terms of compressibility factor, Z, as a truncated expansion ofmolar density, d: $\begin{matrix}{{Z( {T,P,{composition}} )} = {\frac{\rho_{{idea}\quad 1\quad {gas}}}{\rho_{{real}\quad {gas}}} = {1 + {B_{mix}d} + {C_{mix}d^{2}}}}} & \text{[1-13]}\end{matrix}$

where compressibility factor, Z, is a dimensionless density, d is themolar density (a function of T, P, and composition), and the virialcoefficients, B_(mix) and C_(mix), are complicated functions oftemperature and composition. Since Z and d are both functions oftemperature, T. pressure, P, and composition, the solution of Equation[1-13] is iterative.

The Gross Characterization method determines the virial coefficients,B_(mix) and C_(mix) by modeling the natural gas as a three componentmixture containing carbon dioxide, nitrogen, and an equivalenthydrocarbon component. The equivalent hydrocarbon component is thencharacterized by one of the two inferential variable sets, using them todetermine a single hydrocarbon characterization parameter, H_(CH), whichis the molar ideal gross heating value of the equivalent hydrocarbon[kJ/mol at reference conditions of 25° C. and 0.101325 MPa]. H_(CH) isrelated to H_(n,ref) by the following equation: $\begin{matrix}{H_{CH} = \frac{H_{n,{ref}}}{( {1 - ( \frac{X_{CO2} + X_{N2}}{100} )} )}} & \text{[1-14]}\end{matrix}$

Since H_(n,ref) has already been correlated to S_(std), X_(CO2), andX_(N2) in Equation [1-12], H_(CH) is now known as a function of thepre-selected inferential variables S_(std), X_(CO2), and X_(N2), and canbe used to characterize the equivalent hydrocarbon group for Method 3 ofthe Gross Characterization Method.

The most accurate way to determine H_(CH) is from the detailed gascomposition assay using Equations [1-7] and [1-14], but doing so defeatsthe purpose of gross inferential characterization. However, it is usefulas a mechanism to verify the validity of the inferentialcharacterization approach (i.e., Method 3) using the database of naturalgas compositions in Table 1. Calculating natural gas density using bothapproaches over the entire temperature and pressure range of the A.G.A.Gross Characterization Method of Committee Report 8 provides two sets ofresiduals which agree within about ±0.05%.

Thermodynamic Properties from Sound Speed at Standard Temperature andPressure

FIG. 2 is a flow chart summarizing the method 100 of the presentinvention. The method begins with determining the speed of sound in step120. Once the speed of sound in the gas is determined, it is necessaryto determine a plurality (two or more) of concentrations for variousnon-combustible components which comprise the gas in step 130. Examplesusing the concentrations of carbon dioxide and nitrogen concentrationhave been explicitly detailed. The thermophysical property determinationvia correlation in step 140 is then effected by an empirical process, asdiscussed in detail above.

The inferential correlation for various chemical properties may bedetermined, including mixture molecular weight (M) in step 170,mass-based heating value (H_(m)) in step 180, standard density (ρ_(STD))in step 190, mixture molar ideal gross heating value (H_(n,ref)) in step200. Other properties may also be determined, such as molar specificheat at constant volume (c_(v)), molar specific heat at constantpressure (c_(p)), and the ratio of specific heats κ=c_(p)/c_(v).

As has been noted, the standard heating value (H_(v,std)) can becalculated from the mass-based heating value, H_(m), and the standarddensity, ρ_(STD). This is accomplished in step 220. If the flow rate isdetermined in step 230, then the energy flow rate, which is the productof the standard heating value (H_(v,std)) and the flow rate (Q_(v,std)),can be determined in step 240, as shown above.

In summary, the method for determining thermophysical properties of amulti-component gas described above is based upon measuring the speed ofsound in the natural gas mixture at standard pressure (14.73 psia) andstandard temperature (60° F.), along with the molar concentrations ofnitrogen, X_(N2), and carbon dioxide, X_(CO2). Since the speed of soundat standard pressure and temperature is related to the relative densityof the natural gas mixture (the ratio of the natural gas mixture densityto the air density) at standard pressure and temperature, it can be usedin place of the relative density in the Gross Characterization Method ofthe A.G.A. Report no. 8 equation of state (designated as Method 3 above)to calculate the density and properties of the gas at flowingtemperature and pressure.

Thermodynamic Properties from Sound Speed at Arbitrary Temperature andPressure

FIG. 3 illustrates a method for determining thermodynamic propertiesbased on speed of sound measurements that are not necessarily atstandard pressure and temperature. This method permits an energy metermodule that is compatible with conventional ultrasonic flow meters,which measure the speed of sound in flowing gas at arbitrary (i.e.,pipeline) pressure and temperature.

In step 305, three reference natural gas mixtures G₁, G₂, and G₃ areselected that cover the range of interest in natural gas molecularweight (MW). Each selected gas mixture is a “pure hydrocarbon” gasmixture, in that it comprises hydrocarbon gases only and no diluents. Inother words, their diluent concentrations are 0.0 mole % carbon dioxideand 0.0 mole % nitrogen.

For purposes of example herein, the reference gas mixtures are thoseidentified in Table 1 as ID# 40, 26, and 49. Each of the reference gasmixtures has no nitrogen or carbon dioxide component, and are identifiedas Gas ID#40_(—)00_(—)00=G₁, Gas ID#26_(—)00_(—)00=G₂, and GasID#49_(—)00_(—)00=G₃. The digits _(—)00_(—)00 after the Gas ID# indicatethat the mole % concentrations of carbon dioxide and nitrogen are equalto zero. Using standard calculations known in the art, the reference gasmixtures G₁, G₂, and G₃ are found to have molecular weights ofMW1=16.202, MW2=17.318 and MW3=19.017 respectively.

In step 310, a computer process (which may be a computer program such asSonicWare™ software) is used to calculate values of the density, ρ, andthe speed of sound, S, for a matrix of values of gas temperature, T, andgas pressure, P, for G₁, G₂, and G₃. For example, the temperature valuesmay range from about 20° F. to about 150° F. in steps of 10° F. Pressurevalues may include 14.73 psia, 50 psia, 100 psia, and a range of stepsfrom 100 psia to 1400 psia, in steps of 100 psia. The calculated valuesof ρ, S, T, and P are formatted as a look up table, which is then storedin the processor 80 memory.

The subject of the method of FIG. 3 is a natural gas mixture, G_(x),(whose composition is to be estimated) flowing in a pipeline. In Step315, the pressure and temperature of the subject gas mixture aremeasured and recorded as P_(flow), and T_(flow).

In Step 320, the recorded data for the three reference gas mixtures G₁,G₂, and G₃ is used with a cubic-spline fit algorithm to providenumerical (approximate) fits for density and the speed of sound as afunction of temperature and pressure, using the measured values P_(flow)and T_(flow). The result is three pairs of values for the speed of soundand density (S₁, ρ1), (S₂, ρ2) and (S₃, ρ3) corresponding to the threepure hydrocarbon gas mixtures G₁, G₂, and G₃ at the measured P_(flow)and T_(flow).

The use of a look up table and interpolation, as performed in Steps 310and 320, permits the calculations to be done prior to the run timeprocess of FIG. 3. However, as an alternative to Steps 310 and 320, thevalues for the speed of sound and density may be calculated “on thefly”. This requires that appropriate software, such as Sonic Ware, or asimilar program, be embedded (or be otherwise available) during theprocessing for the method of FIG. 3.

In step 330, the variation of density with speed of sound for a purehydrocarbon natural gas mixture is modeled using a second order(quadratic) equation. The form of the equation is: $\begin{matrix}{\rho_{PH} = {{AP} + \frac{BP}{S} + \frac{CP}{S^{2}}}} & \text{[1-18]}\end{matrix}$

, where S is the speed of sound. The values of the coefficients, AP, BP,and CP are determined from the values of (S₁, ρ1), (S₂, ρ2) and (S₃, ρ3)(derived as discussed above in connection with in step 320 byinterpolation or calculation).

The variation in molecular weight of the reference gas mixtures as afunction of speed of sound is also modeled as a second order equation.The form of this equation is: $\begin{matrix}{{M\quad W_{PH}} = {{AMW} + \frac{BMW}{S} + \frac{CMW}{S^{2}}}} & \text{[1-19]}\end{matrix}$

The values of the coefficients AMW, BMW and CMW are then determinedusing the known/derived values of (MW1, S₁), (MW2, S₂) and (MW3, S₃) forthe gas mixtures G₁, G₂, and G3. The molecular weight values werecalculated in step 305. The fully determined equations [1-19] and[1-1-19] now provide a mechanism to calculate, at arbitrary temperatureand pressure, the density and the molecular weight of a natural gasmixture that does not contain diluents.

To account for the effect of natural gas diluents on density andmolecular weight, additional reference data, representative of gasmixtures containing nitrogen and carbon dioxide, must be developed.Therefore, in Step 340, six additional reference gas mixtures areselected. These may be based upon varying the compositions of the purehydrocarbon mixtures identified as ID#40_(—)00_(—)00=G₁, GasID#26_(—)00_(—)00=G₂, and Gas ID#49_(—)00_(—)00=G₃ are selected. Forexample, using the gas mixture designated Gas ID#40_(—)00_(—)00, theconcentration of each hydrocarbon gas component in the Table 1 ID# 40gas mixture can be multiplied by a factor of 0.94, and 2.0 mole % ofcarbon dioxide and 4.0 mole % of nitrogen can be added to form a new(dilute) gas mixture, designated as Gas ID#40_(—)02_(—)04=G₄. Anothergas mixture can be formed by replacing the 2.0 mole % of carbon dioxidewith 4.0 mole % of carbon dioxide, and the 4.0 mole % of nitrogen with2.0 mole % of nitrogen (using the same multiplication factor 0.94 toscale down the original mixture). This produces a new dilute gas mixtureidentified as Gas ID#40_(—)04_(—)02=G₅. In a similar manner, four moregas mixtures identified as Gas ID#26_(—)02_(—)04=G₆, GasID#26_(—)04_(—)02=G₇, Gas ID#49_(—)02_(—)04=G₈, and GasID#49_(—)04_(—)02=G₉ can be formulated. The molecular weights for thesesix new gas mixtures can be calculated using standard methods known inthe art. The resulting MW values for the mixtures G₄, G₅, G₆, G₇, G₈,and G₉ are MW4=17.230, MW5=17.550, MW6=18.280, MW7=18.600, MW8=19.876and MW9=20.196, respectively.

In step 360, a computer process (similar or identical to SonicWare™software) is used to calculate values of the gas mixture density, ρ, andthe speed of sound, S, over a range of values for gas temperature, T,and gas pressure, P, for the gas mixtures G₄ G₅, G₆, G₇, G₈, and G₉ Step360 is similar to step 310. The values of ρ, S, T, and P are then savedas six data files in the processor 80 memory.

In Step 370, in a manner similar to Step 320, the stored data files forgas mixtures G₄, G₅, G₆, G₇, G₈, and G₉ are used to provide numericalfits for density and speed of sound as functions of temperature andpressure in the vicinity of the measured P_(flow) and T_(flow), using acubic-spline fit algorithm. The result is six pairs of speed of soundand gas mixture density values (S₄,ρ4), (S₅,ρ5), (S₆,ρ6), (S₇,ρ7),(S₈,ρ8) and (S₉,ρ9) corresponding to the diluted gas mixtures G₄, G₅,G₆, G₇, G₈, and G₉.

As with Steps 310 and 320, Steps 360 and 370 can be eliminated bydirectly calculating density and speed of sound at the measuredtemperature and pressure. As discussed above, the direct calculationrequires the use of appropriate software rather than look up tables andinterpolation.

In Step 380, the model equations for ρ_(pH) and MW_(pH) are modified toinclude the effect of the natural gas diluents N2 and CO2. The new formof the equation for ρ is: $\begin{matrix}{\rho_{dil} = {( {{AP} + \frac{BP}{S} + \frac{CP}{S^{2}}} )*( {1 + {( {{D1} + \frac{D2}{S} + \frac{D3}{S^{2}}} )*{XCO}_{2}} + {( {{E1} + \frac{E2}{S} + \frac{E3}{S^{2}}} )*{XN}_{2}}} )}} & \text{[1-20]}\end{matrix}$

, where XCO₂ is the mole % of carbon dioxide and XN₂ is the mole % ofnitrogen. The values of AP, BP and CP were determined in step 330. Thesix new coefficients, D1, D2, D3 and E1, E2 and E3 can be calculated bysubstituting the pairs of values (S₄,ρ4) through (S₉,ρ9) into thisequation and using matrix algebra operations to solve the sixsimultaneous equations for the coefficients. The resultingfully-determined equation [1-20] models density as a function oftemperature, pressure, speed of sound, mole % of carbon dioxide, andmole % of nitrogen measured at flowing pressure and temperature.

Similarly, an equation for molecular weight as a function of the speedof sound and the mole % concentrations of carbon dioxide and nitrogencan be formed as follows: $\begin{matrix}{{M\quad W_{dil}} = {( {{AMW} + \frac{BMW}{S} + \frac{CMW}{S^{2}}} )*( {1 + {( {{DM1} + \frac{DM2}{S} + \frac{DM3}{S^{2}}} )*{XCO}_{2}} + {( {{EM1} + \frac{EM2}{S} + \frac{EM3}{S^{2}}} )*{XN}_{2}}} )}} & \text{[1-21]}\end{matrix}$

The values of coefficients AMW, BMW and CMW were determined in step 330.The six new coefficients, DM1, DM2, DM3, EM1, EM2 and EM3 can becalculated by substituting the pairs of values (S₄,MW4) through (S₉,MW9)into equation [1-21] and using matrix algebra operations to solve thesix simultaneous equations. The resulting fully-determined equation[1-21] models molecular weight as a function of temperature, pressure,the speed of sound, the mole % concentration of carbon dioxide, and mole% concentration of nitrogen, measured at flowing pressure andtemperature.

In Step 390, the speed of sound S_(std) at standard pressure andtemperature is calculated. The values of MW for the nine natural gasmixtures (three without diluents, and six with diluents) were determinedin steps 305 and 340. Computer programming (such as SonicWare™ software)is used to calculate the speed of sound and the density for each of thenine reference mixtures, G₁ . . . G₉ at standard pressure andtemperature. The values of AMW, BMW and CMW are determined as in step330, except that the newly-determined sound speed values are used forthe natural gas mixtures without diluents, G₁, G₂ and G₃. Values forDM1, DM2, DM3, EM1, EM2 and EM3 can then be determined as in step 380using the newly-determined sound speed values for the six natural gasmixtures with diluents, G₄, G₅, G₆, G₇, G₈, and G₉. Finally, the valueof molecular weight determined in Step 380 is set equal to the modelequation [1-21] using the coefficients calculated with thenewly-determined sound speed values. The resulting equation can besolved iteratively for the standard sound speed at standard temperatureand pressure for the gas mixture of unknown composition.

In Step 400, the mole % concentrations of carbon dioxide, X_(CO2), andnitrogen, X_(N2), are determined. These concentrations can be measureddirectly or obtained indirectly, as discussed above.

In Step 410, with the value of the standard sound speed now determined,together with the mole % concentrations of carbon dioxide, X_(CO2), andnitrogen, X_(N2), the inferential correlation equation [1-10] can beused to calculate the mass-based heating value, H_(m), in units ofBtu/lbm. The energy flow rate can be calculated by multiplying the valueof the mass-based heating value H_(m) by the natural gas mixture densityderived in step 380, and the gas volumetric flow rate, as measured by aconventional flow meter. Since natural gas transmission companies preferreporting the energy content of natural gas in units of Btu/standardcubic feet of volume, this quantity can be calculated as the product ofthe mass-based heating value H_(m) multiplied by the “standard” density,ρ_(std), which is in turn calculated using the inferential correlationequation [1-11].

FIG. 4 is an alternative illustration of the method of FIG. 3. FIG. 4 isa data flow type diagram. The emphasis is on the various calculationsmade after Steps 305-370, which involve obtaining the data for the ninereferences gases (three without diluents and six with diluents. Asindicated, the density and molecular weight of the subject natural gas,which has an arbitrary temperature and pressure, can be estimated.Further energy measurement can then be calculated by using the estimatedmolecular weight and by calculating speed of sound at standardtemperature and pressure. The calculation of the speed of sound atstandard temperature and pressure is based on the same model equation aswas used to find molecular weight and density, but now with speed ofsound as the unknown value.

Other Embodiments

Many variations and modifications may be made to the disclosedembodiments of the invention without departing from the spirit andprinciples described herein. All such modifications and variations areintended to be included within the scope of the present invention, asdefined by the following claims.

What is claimed is:
 1. A method to determine density of a subject gasmixture having both hydrocarbon and diluent components, having knowntemperature, pressure, speed of sound and known diluent concentrationvalues, comprising the steps of: selecting a set of reference gasmixtures, at least some of the gas mixtures containing diluents;determining the density and speed of sound for each of the reference gasmixtures, at the known temperature and pressure, thereby obtaining a setof reference speed of sound and reference density values; modeling thedensity of a gas mixture as a function of its speed of sound, a numberof coefficients, and its concentrations of diluent components, therebyobtaining a model equation; using the reference speed of sound andreference density values to determine values for the coefficients;substituting the coefficient values and the speed of sound andconcentration values of the subject gas into the model equation; andsolving the model equation to determine the density of the subject gasmixture.
 2. The method of claim 1, wherein one of the diluent componentsis carbon dioxide.
 3. The method of claim 1, wherein one of the diluentcomponents is nitrogen.
 4. The method of claim 1, wherein the step ofdetermining the density and speed of sound of the reference gas mixturesis achieved by interpolating stored values of density and speed of soundover a range of temperature and pressure values.
 5. The method of claim1, wherein the step of determining the density and speed of sound of thereference gas mixtures is achieved by calculating those values at theknown pressure and temperature.
 6. A method to determine the molecularweight of a subject gas mixture having both hydrocarbon and diluentcomponents, and having known temperature, pressure, speed of sound, anddiluent concentration values, comprising the steps of: selecting a setof reference gas mixtures, at least some of the gas mixtures containingdiluents; determining the density and speed of sound for each of thereference gas mixtures, at the known temperature and pressure, therebyobtaining a set of reference speed of sound and reference densityvalues; modeling the molecular weight of a gas mixture as a function ofits speed of sound, a number of coefficients, and its concentrations ofdiluent components, thereby obtaining a model equation; using thereference speed of sound and reference density values to determinevalues for the coefficients; substituting the coefficient values and thespeed of sound and concentration values of the subject gas into themodel equation; and solving the model equation to determine themolecular weight of the subject gas mixture.
 7. The method of claim 6,wherein one of the diluent components is carbon dioxide.
 8. The methodof claim 6, wherein one of the diluent components is nitrogen.
 9. Themethod of claim 6, wherein the step of determining the density and speedof sound of the reference gas mixtures is achieved by interpolatingstored values of density and speed of sound over a range of temperatureand pressure values.
 10. The method of claim 6, wherein the step ofdetermining the density and speed of sound of the reference gas mixturesis achieved by calculating those values at the known pressure andtemperature.
 11. A method to determine a heating value of a subject gasmixture having both hydrocarbon and diluent components, and having knowntemperature, pressure, speed of sound, and diluent concentration values,comprising the steps of: selecting a set of reference gas mixtures, atleast some of the gas mixtures containing diluents; determining thedensity and speed of sound for each of the reference gas mixtures, atthe known temperature and pressure, thereby obtaining a set of referencespeed of sound and reference density values; modeling the molecularweight of a gas mixture as a function of its speed of sound, a number ofcoefficients, and its concentrations of diluent components, therebyobtaining a model equation; using the reference speed of sound andreference density values to determine values for the coefficients;substituting the coefficient values and the speed of sound andconcentration values of the subject gas into the model equation; solvingthe model equation to determine the molecular weight of the subject gasmixture; determining the speed of sound of the subject gas at standardtemperature and pressure; modeling a heating value of the subject gas asa function of its speed of sound at standard temperature and pressure, anumber of coefficients, and its concentrations of diluent components,thereby obtaining a model equation; and solving the model equation todetermine the heating value of the subject gas mixture.
 12. The methodof claim 11, wherein the heating value is a mass based heating value.13. The method of claim 11, wherein one of the diluent components iscarbon dioxide.
 14. The method of claim 11, wherein one of the diluentcomponents is nitrogen.
 15. The method of claim 11, wherein the step ofdetermining the density and speed of sound of the reference gas mixturesis achieved by interpolating stored values of density and speed of soundover a range of temperature and pressure values.
 16. The method of claim11, wherein the step of determining the density and speed of sound ofthe reference gas mixtures is achieved by calculating those values atthe known pressure and temperature.
 17. The method of claim 11, furthercomprising the steps of determining the mass flow rate of the subjectgas mixture, and of using the mass flow rate and heating value tocalculate the energy flow rate of the subject gas mixture.
 18. A methodto determine density, at standard temperature and pressure, of a subjectgas mixture having both hydrocarbon and diluent components, and havingknown temperature, pressure, speed of sound, and diluent concentrationvalues, comprising the steps of: selecting a set of reference gasmixtures, at least some of the gas mixtures containing diluents;determining the density and speed of sound for each of the reference gasmixtures, at the known temperature and pressure, thereby obtaining a setof reference speed of sound and reference density values; modeling themolecular weight of a gas mixture as a function of its speed of sound, anumber of coefficients, and its concentrations of diluent components,thereby obtaining a model equation; using the reference speed of soundand reference density values to determine values for the coefficients;substituting the coefficient values and the speed of sound andconcentration values of the subject gas into the model equation; solvingthe model equation to determine the molecular weight of the subject gasmixture; determining the speed of sound of the subject gas at standardtemperature and pressure; modeling the density, at standard temperatureand pressure, of the subject gas as a function of its speed of sound atstandard temperature and pressure, a number of coefficients, and itsconcentrations of diluent components, thereby obtaining a modelequation; and solving the model equation to determine the density, atstandard temperature and pressure, of the subject gas mixture.
 19. Themethod of claim 18, wherein one of the diluent components is carbondioxide.
 20. The method of claim 18, wherein one of the diluentcomponents is nitrogen.
 21. The method of claim 18, wherein the step ofdetermining the density and speed of sound of the reference gas mixturesis achieved by interpolating stored values of density and speed of soundover a range of temperature and pressure values.
 22. The method of claim18, wherein the step of determining the density and speed of sound ofthe reference gas mixtures is achieved by calculating those values atthe known pressure and temperature.